1. Field of the Invention
The present invention relates generally to a hydraulic power transmission joint for use in automotive 4 wheel-drive mechanisms for the distribution of driving force, and more particularly to a hydraulic power transmission joint designed to suppress cavitation noises arising from a drain mechanism built in a valve block within the joint.
2. Description of the Related Arts
Such a hydraulic power transmission joint for use in an automotive 4 wheel-drive mechanism is hitherto known from U.S. Pat. Nos. 5,706,658 and 5,983,635.
This hydraulic power transmission joint comprises a housing disposed between input and output shafts that are capable of relative rotations, the housing being coupled to one of the input and output shafts and having a cam face formed on its inner side surface; a rotor coupled to the other of the input and output shaft and being rotatably accommodated in the housing, the rotor having a plurality of axially extending plunger chambers; a plurality of plungers each being reciprocatively accommodated in each of the plurality of plunger chambers under a biasing force of the return spring, the plurality of plungers being operated by the cam face upon the relative rotations of the input and output shafts; a discharge hole formed in the rotor and leading to the plurality of plunger chambers; and a valve block having a high-pressure chamber that leads to the discharge hole, the valve block having an orifice for generating a flow resistance under the action of flow of oil discharged by operations of the plurality of plungers.
FIG. 1 shows by way of example a drain mechanism incorporated in a valve block of a hydraulic power transmission joint being currently developed by the present inventors on the basis of such U. S. Patents, the drain mechanism serving also as a torque limiter mechanism. The drain mechanism is generally designated at 101 and includes a drain plug 102 accommodated in an accommodation hole of the valve block not shown, the drain plug 102 being formed with a drain hole 103. The drain plug 102 accommodates a drain pin 104 such that the latter is able to open and close the drain hole 103, the interior of the drain pin 104 being formed with a high-pressure chamber 106 that communicates via a through-hole 105 with a high-pressure chamber located on its left. An orifice 107 is formed in the drain pin 104. The orifice 107 serves to generate a high pressure by a flow resistance occurring when oil from the plungers passes therethrough, to impart it to the plungers so as to allow a transmission of torque. A seal ring 108 is interposed between the drain plug 102 and the drain pin 104. A return spring 110 intervenes between the inner wall of the high-pressure chamber 106 and a limiter plug 109 provided in abutment against the drain plug 102. The drain pin 104 is urged by the return spring 110 to block the drain hole 103. The limiter plug 109 is formed with a communication hole 111 that leads to the high-pressure chamber 106, with a limiter pin 112 adapted to open and close the communication hole 111 being slidably accommodated in the limiter plug 109. The limiter pin 112 is urged via a thermo-switch (not shown) located on its right side by the return spring, to block the communication hole 111.
When the temperature of the thermo-switch reaches a predetermined temperature, the thermo-switch is actuated to release the pressing action on the limiter pin 112 so that the limiter pin 112 can open the communication hole 111 under the hydraulic pressure within the high-pressure chamber 106. For this reason, the hydraulic pressure within the high-pressure chamber 106 of the drain pin 104 becomes zero all at once, allowing the drain pin 104 to open the drain hole 103 under the hydraulic pressure from the high-pressure chamber (not shown) on its left side. Oil within the high-pressure chamber passes through the drain hole 103 and is drained into the low-pressure chamber, ceasing torque transmission to the rear side. By virtue of this, the joint can be prevented from being subjected to a high temperature upon continuous travels over sandy soil, making it possible to avoid any damage to the joint. On the contrary, in the inoperative status of the thermo-switch prior to reaching a predetermined temperature, the hydraulic pressure within the high-pressure chamber 106 of the drain pin 104 becomes larger than the force biasing the limiter pin 112 rightward once a predetermined hydraulic pressure is reached, allowing the limiter 112 to gradually open the communication hole 111. Thus, the drain pin 104 can gradually open the drain hole 103 while keeping its balance. The transmission torque to the rear side is therefore prevented from increasing accordingly as the rotational-speed difference becomes larger, and it is limited to a certain value. This ensures a smooth start of the motor vehicle.
However, in such a hydraulic power transmission joint, the drain pin having an orifice is accommodated in the drain plug while being urged by the return spring, but the drain pin undergoes no positioning in the direction where the orifice rotates. As a result, the cavitation noises may possibly become louder depending on the rotational position of the orifice.
According to an aspect of the present invention there is provided a hydraulic power transmission joint capable of suppressing cavitation noises arising from a drain pin having an orifice in a drain mechanism.
An inventive hydraulic power transmission joint is adapted to be interposed between an input shaft and an output shaft that are rotatable relative to each other, and to transmit torque as a function of the rotational-speed difference between the input and output shafts. The hydraulic power transmission joint comprises a housing coupled to the input shaft and having a cam face formed on its inner side surface; a rotor coupled to the output shaft and being rotatably accommodated in the housing, the rotor having a plurality of axially extending plunger chambers; a plurality of plungers each being reciprocatively accommodated in each of the plurality of plunger chambers under a biasing force of the return spring, the plurality of plungers being operated by the cam face upon the relative rotations of the input and output shafts; a discharge hole formed in the rotor and opening to the plurality of plunger chambers; a valve block coupled to the rotor for being rotated jointly; an orifice formed in the valve block for generating a flow resistance under the action of flow of oil discharged by operations of the plurality of plungers; and a drain mechanism provided in the valve block. The drain mechanism serves, when a predetermined temperature is reached, to open a drain hole to allow oil to flow from a high-pressure side into a low-pressure side to thereby cancel a transmission of torque, the drain mechanism serving, until the predetermined temperature is reached, to regulate the opening of the drain hole to limit a transmission torque to a certain value irrespective of increase in the rotational-speed difference. The drain mechanism includes a drain pin slidably accommodated in a drain plug, and a limiter pin received in a limiter plug disposed in abutment against the drain plug for opening and closing a communication hole that leads to the drain pin, with one end turn portion of the return spring being press fitted into the interior of the drain pin, and with the other end turn portion being press fitted into the interior of the limiter plug, to thereby rotationally position the drain pin.
According to the hydraulic power transmission joint of the present invention having the above configuration, one end turn portion of the return spring is press fitted into the interior of the drain pin having an orifice for generating a flow resistance, with the other end turn portion being press fitted into the interior of the limiter plug to thereby rotationally position the drain pin, whereupon it is possible to restrict the direction of the drain pin having an orifice and thus to reduce the cavitation noises. That is, the press fitting of the return spring end turn portions imparts a whirl-stop function to the return spring so that whirl-stop is provided in the rotational direction to alleviate the cavitation noises without any need for additional pins and grooves.
Herein, the interior of the drain pin may be formed with a flared portion that receives the one end turn portion of the return spring, and the limiter plug may include a groove having a flared portion that receives the other end turn portion of the return spring. Preferably, the end turn portions at opposed ends of the return spring have diameters that increase toward the opposed ends.
The drain mechanism may comprise a limiter plug accommodated within the valve block and having a communication hole through which oil is introduced from a first high-pressure chamber; a thermo-switch located within a low-pressure chamber in the valve block in such a manner as to be urged by a return spring, the thermo-switch having a head pin that protrudes when a predetermined temperature is reached; a fixed pin fixedly inserted into the valve block, the fixed pin being positioned so as to allow the head pin of the thermo-switch urged by the return spring to abut thereagainst, the fixed pin when the head pin protrudes at the predetermined temperature causing the thermo-switch to retreat against the return spring; a limiter pin slidably located within the limiter plug, the limiter pin being pressed by the thermo-switch to close the communication hole for the duration in which the predetermined temperature is not reached, the limiter pin when the predetermined temperature is reached, opening the communication hole as a result of cutoff of the pressing force induced by a retreat of the thermo-switch, the limiter pin when a predetermined torque is reached, opening the communication hole by a high pressure from the first high-pressure chamber; a drain plug that follows the limiter plug, accommodated in the valve block, the drain plug having a drain hole through which oil is discharged from a second high-pressure chamber; and a drain pin slidably located within the drain plug in such a manner as to be urged by a return spring, the drain pin having the first high-pressure chamber into which oil is introduced from the second high-pressure chamber and which opens to the communication hole, the drain pin closing the drain hole under a pressing force of the limiter pin until a predetermined temperature is reached, the drain pin rapidly opening the drain hole in response to a retreat of the limiter pin effected when the predetermined temperature is reached, the drain pin when a predetermined torque is reached for the duration in which the predetermined temperature is not yet reached, opening the drain hole while adjusting its balance so as to keep the predetermined torque. Furthermore, the orifice is preferably formed in a communication passage between the first high-pressure chamber located in the interior of the drain pin and the exterior low pressure side.